The present invention relates to a method of forming an electrode structure including a lower layer of polysilicon or amorphous silicon and an upper layer of a high-melting-point metal, and a method of fabricating a semiconductor device including a gate electrode formed from the electrode structure.
In a conventional MOS transistor, a gate electrode is formed from a polysilicon film. In accordance with improvement of LSIs in refinement and high speed operation, there are increasing demands for lowering the resistance of the gate electrode of a MOS transistor.
Therefore, as technique to lower the resistance of the gate electrode, a polymetal gate electrode having a laminated structure including a lower polysilicon film and an upper high-melting-point metal film has been proposed to be used as the gate electrode, and a tungsten film has been proposed as the upper high-melting-point metal film. By using a tungsten film as the upper high-melting-point metal film, the resistance value of the gate electrode can be lowered.
It is necessary to form a barrier film of tungsten nitride (WNx) or titanium nitride (TiN) between a polysilicon film and a tungsten film in order to prevent an impurity (such as B, P and As) introduced into the polysilicon film from diffusing into the tungsten film (as disclosed in, for example, Japanese Laid-Open Patent Publication No. 11-261059 or 7-235542).
FIG. 12(a) is a sectional view of an electrode structure of a first conventional example. As is shown in FIG. 12(a), a gate electrode is formed on a semiconductor substrate 1 with a gate insulating film 2 sandwiched therebetween, and the gate electrode includes a polysilicon film 3, a barrier film 4A of tungsten nitride (WNx) and a tungsten film 5 successively formed upward.
FIG. 12(b) is a sectional view of an electrode structure of a second conventional example. As is shown in FIG. 12(b), a gate electrode is formed on a semiconductor substrate 1 with a gate insulating film 2 sandwiched therebetween, and the gate electrode includes a polysilicon film 3, a barrier film 4B of titanium nitride (TiN) and a tungsten film 5 successively formed upward.
In the electrode structure of the first conventional example, a heat treatment conducted in a later procedure evaporates nitrogen included in the barrier film 4A of tungsten nitride, so that the barrier film 4A can be changed into the tungsten film 5. In addition, nitrogen included in the barrier film 4A reacts with silicon included in the polysilicon film 3, so that a reaction layer 6 of silicon nitride (SiN) having a very large resistance value can be formed between the polysilicon film 3 and the tungsten film 5 as is shown in FIG. 12(c). As a result, the resistance value of the gate electrode is disadvantageously increased.
Therefore, Japanese Laid-Open Patent Publication No. 7-235542 describes that the sheet resistance of the reaction layer 6 can be reduced to lower the resistance value of the gate electrode by setting the surface density of nitrogen included in the reaction layer 6 of silicon nitride to a predetermined value or less.
The present inventors have found, however, that the resistance value of the gate electrode cannot be lowered even by setting the surface density of nitrogen included in the reaction layer 6 to the predetermined value or less in the electrode structure of the first conventional example.
Therefore, the reason why the resistance value of the gate electrode cannot be lowered in the first example has variously examined to find the following: When the thickness of the barrier film 4A is reduced to approximately 0.1 through 1.0 nm in order to reduce the surface density of nitrogen included in the reaction layer 6, the barrier film 4A cannot exhibit the barrier function. Accordingly, tungsten silicide (WSix) is formed, so that the resistance value of the gate electrode cannot be lowered. When the thickness of the barrier film 4A is increased to exceed 1.0 nm, although the barrier film 4A can exhibit the barrier function, the reaction layer 6 of silicon nitride having a very large resistance value is formed between the polysilicon film 3 and the tungsten film 5. Accordingly, the interface resistance value between the polysilicon film 3 and the tungsten film 5 is increased.
As another problem, since a tungsten nitride film is poor at heat resistance, a great deal of nitrogen included in the tungsten nitride film can be diffused through a heat treatment conducted at a temperature of 750xc2x0 C. or more, so that the tungsten nitride film can be changed into a tungsten film.
In the case where a barrier film of titanium nitride is used as in the second conventional method, the interface resistance value between the polysilicon film 3 and the tungsten film 5 is increased owing to the reaction layer 6 of silicon nitride having a very large resistance value formed between the polysilicon film and the tungsten film for the reason described below.
First, as is shown in FIG. 13(a), a polysilicon film 3 is formed on a semiconductor substrate 1 with a gate insulating film 2 sandwiched therebetween. The polysilicon film 3 is doped with a p-type impurity such as boron when a p-type gate electrode is to be formed, and is doped with an n-type impurity such as phosphorus when an n-type gate electrode is to be formed. Next, in order to deposit a titanium nitride film 4B on the polysilicon film 3, the semiconductor substrate 1 is loaded within a chamber where a titanium target 7 including titanium as a main component is placed, and a mixed gas including argon and nitrogen is introduced into the chamber and discharge is caused within the chamber. In this manner, plasma of the argon gas and the nitrogen gas is generated, so that a reaction layer 6 of silicon nitride can be formed in a surface portion of polysilicon film 3 through a reaction between nitrogen ions of the plasma and silicon of the polysilicon film 3. Furthermore, the titanium target 7 is nitrided so as to form a titanium nitride film 8 thereon, and titanium nitride is sputtered out from the titanium nitride film 8. As a result, the barrier film 4B of titanium nitride is formed on the reaction layer 6 as is shown in FIG. 13(b).
Then, the semiconductor substrate 1 is transferred to a chamber where a tungsten target 9 including tungsten as a main component is placed, and an argon gas is introduced into the chamber and discharge is caused within the chamber. In this manner, plasma of the argon gas is generated, so that tungsten can be sputtered out from the tungsten target 9 through sputtering of argon ions included in the plasma. The sputtered tungsten is deposited on the surface of the titanium nitride film 4B, and thus, a tungsten film 5 is formed on the titanium nitride film 4B as is shown in FIG. 13(c).
Next, an impurity layer to be formed into a source or drain of a MOS transistor is formed in the semiconductor substrate 1, and a heat treatment is carried out for activating the impurity layer at a temperature of, for example, 750xc2x0 C. or more. In this manner, as is shown in FIG. 14(a), excessive nitrogen included in the barrier film 4B is diffused into an upper portion of the polysilicon film 3. As a result, the reaction layer 6 of titanium nitride is increased in its thickness as is shown in FIG. 14(b).
Moreover, the present inventors have examined the relationship between the temperature of the heat treatment and the interface resistance of the barrier film after the heat treatment. FIG. 15 shows the relationship between the temperature (xc2x0C.) of the heat treatment and the interface resistance (Rc) between the polysilicon film and the high-melting-point metal film after the heat treatment. In FIG. 15, a symbol xe2x97xaf indicates the result obtained when a barrier film of tungsten nitride (WNx) is formed on an n-type polysilicon film (indicated as NPS); a symbol ◯ indicates the result obtained when a barrier film of tungsten nitride is formed on a p-type polysilicon film (indicated as PPS); a symbol ♦ indicates the result obtained when a barrier film of titanium nitride (TiN) is formed on an n-type polysilicon film; and a symbol ⋄ indicates the result obtained when a barrier film of titanium nitride is formed on a p-type polysilicon film. Furthermore, the resistance shown in FIG. 15 is not ohmic resistance, and hence, a resistance value obtained by allowing a current of 1 mA/xcexcm2 to flow is shown as interface resistance.
It is understood from FIG. 15 that in using the barrier film 4B of titanium nitride, the interface resistance is high even when the heat treatment is conducted at a low temperature. Furthermore, the present inventors have found through experiments that in using the barrier film 4B of titanium nitride, the interface resistance is high even when the heat treatment is not conducted. This is because the reaction layer 6 of titanium nitride is formed between the polysilicon film 3 and the barrier film 4B as is shown in FIGS. 13(a) through 13(c).
Also, when the barrier film 4A of tungsten nitride is used, although the interface resistance is lower than in using the barrier film 4B of titanium nitride, the interface resistance is abruptly increased through a heat treatment conducted at 750xc2x0 C. or more. This is because nitrogen included in tungsten nitride of the barrier film 4A is diffused through the heat treatment conducted at 750xc2x0 C. or more, so as to form the reaction layer 6 of silicon nitride between the polysilicon film 3 and the tungsten film 5.
As the interface resistance (Rc) between the polysilicon film 3 and the tungsten film 5 is increased, the operation speed of the MOS transistor is lowered. Specifically, when the gate electrode is AC operated, distributed capacity generated in the gate insulating film is repeatedly charged and discharged. Therefore, a current flows through the distributed interface resistance, so that the distributed interface resistance can affect to lower the operation speed of the MOS transistor. When the operation speed of the MOS transistor is lowered, the operation speed of an LSI including the MOS transistor is lowered, which causes a problem that signal delay time is increased. Since the operation speed of an LSI is regarded the most significant these days, degradation in the operation speed of the MOS transistor by merely several % can cause a serious problem.
In consideration of the aforementioned conventional problems, an object of the invention is lowering interface resistance between a polysilicon film and a high-melting-point metal film.
In order to achieve the object, the method of forming an electrode structure of this invention comprises the steps of forming a barrier film on a silicon-containing film including silicon as a main component; depositing a high-melting-point metal film on the barrier film, whereby forming a laminated structure including the silicon-containing film, the barrier film and the high-melting-point metal film; and conducting a heat treatment on the laminated structure at a temperature of 750xc2x0 C. or more, and the step of forming the barrier film includes sub-steps of forming a first metal film of a nitride of a metal on the silicon-containing film; forming, on the first metal film, a second metal film of the metal or the nitride of the metal with a smaller nitrogen content than the first metal film; and forming, on the second metal film, a third metal film of the nitride of the metal with a larger nitrogen content than the second metal film.
In the method of forming an electrode structure of this invention, after conducting the heat treatment, part of nitrogen included in the first metal film and part of nitrogen included in the third metal film is consumed in nitriding of the second metal film, and merely little of the nitrogen included in the first metal film is concerned with nitriding of the silicon-containing film. Accordingly, a reaction layer of a silicon nitride film having a very large resistance value is not formed on the interface between the silicon-containing film and the barrier film, or if the reaction layer is formed, the thickness is small. Accordingly, the interface resistance between the silicon-containing film and the high-melting-point metal film is lowered.
In the method of forming an electrode structure of this invention, the metal is preferably titanium.
In the method of forming an electrode structure, the step of forming the barrier film preferably includes sub-steps of forming the first metal film by depositing the nitride of the metal on the silicon-containing film through sputtering using an inert gas substantially including no nitrogen gas conducted on a nitride film of the metal formed in a surface portion of a target including the metal as a main component, and forming the second metal film by depositing the metal on the first metal film; and conducting sputtering using a mixed gas including a nitrogen gas and an inert gas on the target, whereby the third metal film is formed by depositing, on the second metal film, the nitride of the metal obtained through a reaction of the metal and nitrogen included in the mixed gas.
In this manner, the sputtering using an inert gas substantially including no nitrogen gas is conducted for forming the first metal film. Therefore, a silicon nitride film can be prevented from being formed in a surface portion of the silicon-containing film, and hence, the interface resistance between the silicon-containing film and the high-melting-point metal film is further lowered. Furthermore, the first metal film, the second metal film and the third metal film can be continuously formed by using the same target by merely changing the kind of gas to be used for the sputtering, resulting in improving the throughput.
In this case, the method preferably further comprises, after the step of forming the barrier film, a step of conducting sputtering using an inert gas substantially including no nitrogen gas on the nitride film of the metal formed in the surface portion of the target formed in the sub-step of forming the third metal film.
When the method thus includes the step of conducting sputtering the metal nitride film formed in the surface portion of the target during the formation of the third metal film by using an inert gas substantially including no nitrogen gas, the concentration of nitrogen in the metal nitride film is reduced. Therefore, the nitrogen concentration in the first metal film formed by using the metal nitride film is reduced, resulting in further reducing the nitrogen concerned with the nitriding of the silicon-containing film. Accordingly, the reaction layer of a silicon nitride film is more difficult to form on the interface between the silicon-containing film and the barrier film, or if it is formed, the thickness is further smaller. As a result, the interface resistance between the silicon-containing film and the high-melting-point metal film can be further lowered.
In the method of forming an electrode structure, the step of forming the barrier film includes preferably sub-steps of introducing an inert gas substantially including no nitrogen gas into a chamber where a target including the metal as a main component and having a nitride film of the metal in a surface portion thereof is placed and causing discharge within the chamber, whereby the first metal film is formed by depositing, on the silicon-containing film, the nitride of the metal sputtered out from the nitride film of the metal, and forming the second metal film by depositing the metal on the first metal film; and introducing a mixed gas including a nitrogen gas and an inert gas into the chamber where the target is placed and causing discharge within the chamber, whereby the third metal film is formed by depositing, on the second metal film, the nitride of the metal obtained through a reaction between the metal and nitrogen included in the mixed gas.
In this manner, an inert gas substantially including no nitrogen gas is introduced into the chamber for forming the first metal film, and hence, a silicon nitride film is not formed in the surface portion of the silicon-containing film. Accordingly, the interface resistance between the silicon-containing film and the high-melting-point metal film can be further lowered. Also, the first metal film, the second metal film and the third metal film can be continuously formed by using the same target placed in the same chamber by merely changing the kind of gas to be used for the sputtering, resulting in improving the throughput.
In this case, the method preferably further comprises, after the step of forming the barrier film, a step of introducing an inert gas substantially including no nitrogen gas into the chamber and causing discharge within the chamber.
When the method thus includes, after the step of forming the barrier film, the step of introducing an inert gas substantially including no nitrogen gas into the chamber and causing discharge within the chamber, the metal nitride film formed in the surface portion of the target during the formation of the third metal film can be sputtered by using the inert gas substantially including no nitrogen gas. Accordingly, since the concentration of nitrogen in the metal nitride film is reduced, the nitrogen concentration in the first metal film formed by using the metal nitride film can be reduced, and hence, further less nitrogen is concerned with the nitriding of the silicon-containing film. As a result, the reaction layer of a silicon nitride film is more difficult to form on the interface between the silicon-containing film and the barrier film, or if the reaction layer is formed, the thickness can be further smaller. Therefore, the interface resistance between the silicon-containing film and the high-melting-point metal film can be further lowered.
The method of fabricating a semiconductor device of this invention comprises the steps of forming a polysilicon film on a semiconductor region; forming a barrier film on the polysilicon film; depositing a high-melting-point metal film on the barrier film, whereby forming a gate electrode including the silicon-containing film, the barrier film and the high-melting-point film; forming an impurity layer serving as a source or drain through ion implantation of an impurity into the semiconductor region with the gate electrode used as a mask; and activating the impurity layer by conducting a heat treatment at a temperature of 750xc2x0 C. or more, and the step of forming the barrier film includes sub-steps of forming a first metal film of a nitride of a metal on the silicon-containing film; forming, on the first metal film, a second metal film of the metal or the nitride of the metal with a smaller nitrogen content than the first metal film; and forming, on the second metal film, a third metal film of the nitride of the metal with a larger nitrogen content than the second metal film.
In the present method of fabricating a semiconductor device, a semiconductor device is fabricated by using the method of forming an electrode structure of this invention. Therefore, even when the heat treatment is carried out at 750xc2x0 C. or more for activating the impurity layer serving as a source or drain, the interface resistance between the polysilicon film and the high-melting-point metal film in the gate electrode can be low. As a result, the delay time of the MOS transistor can be reduced, resulting in improving the operation speed of the MOS transistor.
In the method of fabricating a semiconductor device of this invention, the metal is preferably titanium.